Taxonomy is the practice and science of classification. A taxonomy is a collection of controlled vocabulary terms organized into a hierarchical structure. Each term in a taxonomy can be in one or more parent/child (broader/narrower) relationships to other terms in the taxonomy. A taxonomy can be created with a particular set of applications in mind. A particular application that is based on a particular taxonomy can potentially be enriched by using other taxonomies. Integration of two or more taxonomies into a single taxonomy, however, is a challenge, largely because the perspectives of the individual taxonomies differ and a larger vocabulary is needed to reconcile very different taxonomies merging, which may create a lot of gaps, creating a tendency to merge nearly similar taxonomies (a similar taxonomy may often not be available).